The present invention relates to a golf club head with a vibration absorber capable of absorbing unpleasant vibrations on off-center hits without absorbing agreeable vibrations on on-center hits.
In the U.S. Patent Application Publication No. 2007-129165-A1, there is disclosed a golf club head provided with a vibration absorber to absorb vibration of a golf club head at impact, wherein the absorber is made of a first visco-elastic material and a second visco-elastic material whose loss coefficient (loss tangent) has a different temperature dependency than that of the first visco-elastic material so as to cover a wide frequency range of the vibrations.
On the other hand, it is empirically well known to the golfers that, in the case of good shots such that the ball is hit at the sweet spot of the club face (namely, on-center hit), the player feels solid agreeable vibration at the hands through the grip, but in the case of miss shots such that the ball is hit off the sweet spot (namely, off-center hit), the player feels dull unpleasant vibration and sometimes numbness at the hands.
Thus, the vibration include the agreeable vibrations on on-center hits, and the unpleasant vibrations on off-center hits.
From various test results, it was confirmed that the vibrations on on-center hits are higher in the frequency than the vibrations on off-center hits.
As in the above-mentioned prior art, if a plurality of visco-elastic materials different in the loss coefficient are used in one golf club head in order to absolve vibrations of a wide frequency range, then not only the unpleasant vibrations on off-center hits but also the agreeable vibrations on on-center hits are absorbed. Therefore, the impact feeling is blurred and considered as being not good for advanced golfers in particular, and as a result, the golf club head is felt by the player as being difficult to control the ball.
The present inventor quantitatively investigated the vibrations of a golf club head at impact as follows: an iron-type club head with an acceleration pickup attached to the hosel portion was suspended by a thread; the club face was hit with a hammer to apply an impact; the acceleration was measured at the hosel portion by the use of the acceleration pickup; and the impulse response (frequency transfer function) of the club head was measured with respect to various hitting positions of the club face.
From the results of the above-mentioned experimentation, it was discovered that the frequency spectrum of the vibration when hit the ball at the sweet spot has a peak at a frequency of about 3 kHz, whereas the frequency spectrum of the vibration when hit the ball off the sweet spot has a peak at a frequency of lower than about 2 kHz.
Therefore, in order for the unpleasant vibrations on off-center hits to be reduced effectively while keeping the agreeable vibrations on on-center hits as much as possible, it is important that the vibration absorber is made from a material whose vibration absorbing power is low around 3 kHz but high around 2 kHz or less.
The present inventor further studied and discovered that such desirable vibration absorbing power can be obtained by controlling or adjusting the temperature dependency of the loss tangent of a visco-elastic material. More specifically, the temperature at which the loss tangent becomes its maximum value is set in a specific high-temperature range, and the loss tangent at 30 deg.C. is increased to a specific value, and the loss tangent at −40 deg.C. is decreased to a specific value.